Technical Field
The present invention is related to a Discovery Signal (DS) transmission scheme in D2D system, and more particular to a DS transmission scheme in Device-to-Device (D2D) configuration based on Long Term Evolution (LTE).
Related Art
The traditional LTE (Long Term Evolution) system of 3rd Generation Partner Project (3GPP) defines two frame structures, that is frame structure 1 of Frequency Division Duplex (FDD) system and frame structure 2 of Time Division Duplex (TDD) system. The difference between FDD and TDD lies in that each subframe of FDD frame structure is 1 millisecond, while the TDD system defines 1 to 2 special subframes in one frame (10 subframes). The special subframes are composed of a downlink synchronous time slot, a guard period and an uplink synchronization time slot.
In the traditional 3GPP version, the data transfer occurs between the base station and the user equipment (UE). In 3GPP R12, Device-to-Device (D2D) communication is approved and discussed. The essential characteristic of D2D is to allow data transfer between the UEs. For FDD and TDD system, 3GPP has concluded that the UE is not allowed to simultaneously receive and transmit in D2D system in the seventy-third meeting of Radio Access Network Working Group 1 (RAN1 #73). Furthermore, for avoiding the interference on D2D communication brought by the downlink data, the UE occupies the traditional uplink resource, i.e., the uplink frequency band of FDD and the uplink subframes of the subframes (TDD) are used for D2D communication (the TDD downlink subframe needs further discussion).
In the D2D system, the UE triggers the establishment of the D2D connection via receiving or transmitting Discovery Signal (DS). Besides indicating the existence of the D2D UE, DS may further carry information bits. According to the conclusion reached in the RAN1 #74 meeting of 3GPP, DS may be one or two of the following:                DS may include information composed by X information bits;        DS is a characteristic sequence, wherein the characteristic sequence is based on Physical Random Access Channel (PRACH), Sounding Reference Signal (SRS), and Primary/Secondary Synchronization Sequence (PSS/SSS). The PSS is a Zadoff-Chu sequence. The SSS is a pseudo-random sequence.        
If the data packages composed by information bits are adopted, DS may carry more information such that the receiving UE of DS may directly initiate D2D communication without further information. However, the time synchronization and frequency synchronization requirement is higher for the way of adopting the data packages composed by information bits.
If the characteristic sequence is adopted, DS may merely carry less information such that the further information is likely to be required for the receiving UE of DS to initiate D2D communication. The time synchronization and frequency synchronization requirement is lower for the characteristic sequence.
No matter which approach is adopted, a solution is required for the information carried by DS to minimize the required information bits or candidate characteristic sequences. The present invention discloses a DS solution in D2D system.